Mycobacteria are a genus of aerobic intracellular bacterial organisms that, upon infection of a host, survive within endosomal compartments of monocytes and macrophages. Human mycobacterial diseases include tuberculosis (caused by M. tuberculosis), leprosy (caused by M. leprae), Bairnsdale ulcers (caused by M. ulcerans), and various infections caused by M. marinum, M. kansasii, M. scrofulaceum, M. szulgai, M. xenopi, M. fortuitum, M. chelonei, M. haemophilum and M. intracellulare (see Wolinsky, E., Chapter 37 in Microbiology: Including Immunology and Molecular Genetics, 3rd Ed., Harper & Row, Philadelphia, 1980).
One third of the world's population harbors M. tuberculosis and is at risk for developing tuberculosis (TB). Young children bear the burden of developing tuberculosis (TB) disproportionately. Once infected, children are not only more susceptible to TB than adults, but also are more likely to develop a severe form of the disease. Specifically, following infection more than 90% of immunocompetent adults will establish an asymptomatic, latent TB infection (LTBI), which carries a 5-10% life-time risk of reactivation disease. Whereas, in the majority of young infants, a primary Mtb infection will advance to active TB, and then in a substantial proportion of those with active TB, the disease will progress to a more severe form (e.g., military TB). In addition to an increased susceptibility to TB, prompt diagnosis in children is complicated by the fact that children with progressive primary infections seldom present with a positive sputum acid-fast bacillus smear, which is commonly seen in adult pulmonary reactivation disease. Early detection is essential since progression of the disease occurs during the period of diagnostic delay.
In immunocompromised patients, tuberculosis is increasing at a nearly logarithmic rate, and multidrug resistant strains are appearing. In addition, Mycobacterial strains which were previously considered to be nonpathogenic strains (e.g., M. avium) have now become major killers of immunosuppressed AIDS patients. Moreover, current Mycobacterial vaccines are either inadequate (such as the BCG vaccine for M. tuberculosis) or unavailable (such as for M. leprae) (Kaufmann, S., Microbiol. Sci. 4:324-328, 1987; U.S. Congress, Office of Technology Assessment, The Continuing Challenge of Tuberculosis, pp. 62-67, OTA-H-574, U.S. Government Printing Office, Washington, D.C., 1993).
Inhibiting the spread of tuberculosis requires effective vaccination and accurate, early diagnosis of the disease. Currently, vaccination with live bacteria is the most efficient method for inducing protective immunity. The most common Mycobacterium employed for this purpose is Bacillus Calmette-Guerin (BCG), an avirulent strain of Mycobacterium bovis. However, the safety and efficacy of BCG is a source of controversy and some countries, such as the United States, do not vaccinate the general public.
Diagnosis of tuberculosis is commonly achieved using a skin test, which involves intradermal exposure to tuberculin PPD (protein-purified derivative). Antigen-specific T cell responses result in measurable induration at the injection site by 48 to 72 hours after injection, which indicates exposure to Mycobacterial antigens. However, the sensitivity and specificity of this test are not ideal; individuals vaccinated with BCG cannot be distinguished from infected individuals. In addition, it is not particularly effective in diagnosing children or LTBI. Accordingly, there is a need in the art for improved diagnostic methods for detecting tuberculosis, specifically for detecting LTBI and for diagnosing TB infections in children.